1. Field of the Invention
The present invention pertains to a propulsion system for an aircraft, and more particularly to method and apparatus for propelling an aircraft using air liquefaction and storage.
The recent discovery of a hole in the ozone layer over Antarctica has stimulated efforts to study the upper atmosphere in Antarctica and around the globe. Such a study entails sampling the atmosphere at a plurality of altitudes from 20 km to 40 km. Currently, balloons and manned aircraft are capable of performing parts of this mission. However, balloons are notoriously difficult to control and their sampling is imprecise and dependent upon the balloon's path. Manned vehicles are very expensive to operate and have many safety constraints, thus reducing the amount of sampling that canbe carried out.
Recently, it has been proposed to utilize a remotely piloted vehicle (RPV) to sample the upper atmosphere in a predictable, organized fashion. The field of RPVs is rather well developed, as in evident by U.S. Pat. No. 3,937,424; U.S. Pat. No. 3,957,230; U.S. Pat. No. 4,415,133; and U.S. Pat. No. 4,697,761. However, all known RPVs are incapable of providing an instrument platform capable of spanning the 20-40 km altitude band, and capable of loitering in the area of interest for a long period of time. This is because known propulsion systems for RPVs include internal combustion engines, gas turbines, batteries, etc. These systems are very heavy, requiring a massive air frame. Further, such systems require storage of on-board fuel, severely limiting the mission duration time.
U.S. Pat. No. 4,415,133 depicts a solar powered RPV which eliminates some of the drawbacks noted above. However, a solar powered RPV will be dependent upon weather conditions, and cannot provide the power necessary to boost an instrument payload to the higher appropriate altitudes.
The fundamental constraint in devising a propulsion system for a high-altitude flight in the earth's atmosphere is the rapid reduction in atmospheric density with altitude. The resulting low pressures at high-altitude makes it difficult to sustain the combustion process typically associated with aircraft propulsion devices. One approach is to fly faster than the speed of sound (supersonic or even hypersonic) and use the ram pressure created in an air inlet to provide sufficient internal pressure to support combustion. However, such supersonic or hypersonic flight would severely restrict the type of atmospheric samples which can be obtained. Thus, atmospheric sampling missions generally require subsonic flight speeds in order to avoid passing atmospheric constituents through a shock wave before they can be sampled.
Previous high-altitude subsonic aircraft (as used herein "high-altitude" refers to altitudes generally exceeding 20 km (65,000 ft)) have used either turbojet propulsion (as in the Lockheed U-2, TR-1, and ER-2 aircraft) or internal combustion with several stages of turbo-machinery to provide the desired inlet air pressure. The most notable example of this is the Teledyne-Continental engine developed for the Boeing Condor, which set an unmanned altitude record of 67,000 ft in 1988.
A variety of unconventional propulsion systems have been proposed. During the 1970's NASA developed an unmanned aircraft called the Mini-Sniffer using a monopropellant (hydrazine) to achieve altitudes up to 98,000 ft. This program was cancelled before altitudes exceeding 20,000 ft were demonstrated. Electropropulsion systems using solar cells have been proposed, but the current state of the art in the solar cells is such that the power-to-weight ratio severely restricts the wing loadings of available vehicles. Electric propulsion using batteries has been proposed, but the weight of the batteries prohibits operation at high altitudes or long mission durations. Electropropulsion using lithium peroxide fuel cells has been proposed for helicopter-type vehicles (see U.S. Pat. No. 4,709,882). However, since the lithium peroxide cells carry all of the necessary oxygen on board, they are excessively heavy and are unsuitable for long-duration flight.
The 1986 discovery of the Antarctic ozone hole has lead to an increased interest in the chemistry and dynamics of the upper atmosphere. Although existing vehicles such as manned aircraft and unmanned balloons have been pressed into service, the dual requirements of high altitude and long range cannot be met by any existing aircraft or propulsion system. A variety of new scientific missions are under consideration. A typical mission is shown in FIG. 1 and involves cruise ranges on the order of 10,000 km and altitude excursions of up to 40 km.
Thus, what is required in a high-altitude atmosphere-sampling RPV is a propulsion system that offers a range potential in excess of 10,000 km, an altitude potential of up to 40 km, the ability to remain subsonic throughout its mission profile, a non-polluting operation, and sufficient scaling flexibility so that it can be applied to a range of aircraft including small unmanned platforms with payloads of approximately 50 kg to large manned platforms carrying payloads in excess of 1000 kg.